A group III-V compound semiconductor containing nitrogen (group III nitride semiconductor) has a bandgap corresponding to the energy of light having a wavelength in the infrared to ultraviolet region, and thus is useful as a material for a light-emitting device which emits light having a wavelength in the infrared to ultraviolet region and a light-receiving device which receives light having a wavelength in that region.
In addition, the group III nitride semiconductor has strong binding between atoms which constitute the group III nitride semiconductor, high breakdown voltage, and high saturation electron speed. Therefore, the group III nitride semiconductor is also useful as a material for an electronic device such as a high-temperature-resistant, high-power, and high-frequency transistor.
Furthermore, the group III nitride semiconductor is also attracting attention as a material which hardly damages the environment and is easy to handle.
In order to fabricate a practical nitride semiconductor device using the group III nitride semiconductor which is an excellent material as described above, it is necessary to stack a group III nitride semiconductor layer made of a thin film of the group III nitride semiconductor on a predetermined substrate to form a predetermined device structure.
As the substrate, it is most suitable to use a substrate made of a group III nitride semiconductor having a lattice constant and a thermal expansion coefficient which allow the group III nitride semiconductor to grow directly on the substrate. For example, a gallium nitride (GaN) substrate or the like is preferably used as the substrate made of the group III nitride semiconductor.
At present, however, the GaN substrate has small dimensions, that is, has a diameter of two inches or less, and is very expensive. Therefore, the GaN substrate is not practical.
For this reason, at present, a sapphire substrate, a silicon carbide (SiC) substrate, or the like having a large difference in lattice constant and thermal expansion coefficient from the group III nitride semiconductor is used as a substrate for fabricating a nitride semiconductor device.
There exists a difference in lattice constant of about 16% between the sapphire substrate and GaN, which is a typical group III nitride semiconductor. In addition, there exists a difference in lattice constant of about 6% between the SiC substrate and GaN. When such a large difference in lattice constant exists between the substrate and the group III nitride semiconductor which grows thereon, it is generally difficult to epitaxially grow a crystal made of the group III nitride semiconductor on the substrate. For example, when a GaN crystal is epitaxially grown directly on the sapphire substrate, there is a problem that three-dimensional growth of the GaN crystal cannot be avoided and a GaN crystal having a flat surface cannot be obtained.
Thus, a layer referred to as a so-called buffer layer for eliminating the difference in lattice constant between the substrate and the group III nitride semiconductor is generally formed between the substrate and the group III nitride semiconductor.
For example, Patent Literature 1 (Japanese Patent No. 3026087) describes a method for forming a buffer layer of aluminum nitride (AlN) on a sapphire substrate by a Metal-Organic Vapor Phase Epitaxy (MOVPE) method, and thereafter growing a group III nitride semiconductor made of AlxGa1-xN.
However, also with the method described in Patent Literature 1, it has been difficult to obtain a buffer layer of AlN having a flat surface, with good reproducibility. This is considered to be because, when the buffer layer of AlN is formed by the MOVPE method, trimethyl aluminum (TMA) gas and ammonia (NH3) gas used as source gases easily react in a vapor phase.
Accordingly, in the method described in Patent Literature 1, it has been difficult to grow a high-quality group III nitride semiconductor made of AlxGa1-xN which has a flat surface and low defect density, on the buffer layer of AlN, with good reproducibility.
In addition, for example, Patent Literature 2 (Japanese Patent Publication No. 5-86646) discloses a method for forming an AlxGa1-xN (0<x≦1) buffer layer on a sapphire substrate by a high-frequency sputtering method which applies a direct current (DC) bias.
However, a group III nitride semiconductor formed on the AlxGa1-xN (0<x≦1) buffer layer by the method described in Patent Literature 2 has not had excellent crystallinity, as described in paragraph [0004] of Patent Literature 3 and paragraph [0004] of Patent Literature 4.
Thus, Patent Literature 3 (Japanese Patent No. 3440873) proposes a method for heat-treating a buffer layer made of a group III nitride semiconductor formed by a DC magnetron sputtering method, under an atmosphere of a mixed gas of hydrogen gas and ammonia gas. In addition, Patent Literature 4 (Japanese Patent No. 3700492) proposes a method for forming a buffer layer made of a group III nitride semiconductor having a film thickness of not less than 50 angstroms and not more than 3000 angstroms, on a sapphire substrate having a temperature raised to 400° C. or more, by the DC magnetron sputtering method.
Further, Patent Literature 5 (Japanese Patent Laying-Open No. 2006-4970) discloses a technique for stacking an Al2O3 layer, an AlOxNy layer, and an AlN layer in this order on a sapphire substrate. These three layers are formed by a reactive sputtering method using ECR (Electron Cyclotron Resonance) plasma, and, in order to alleviate a difference in lattice constant between the sapphire substrate and the AlN layer as the uppermost layer of these three layers, the Al2O3 layer and the AlOxNy layer are inserted between the sapphire substrate and the AlN layer (paragraphs [0019] to [0023] of Patent Literature 5). Patent Literature 5 also proposes a method for further forming a buffer layer made of p-type GaN on the AlN layer by the MOVPE method, and forming a nitride semiconductor layer on the buffer layer (paragraph [0024] of Patent Literature 5).
Furthermore, Patent Literature 6 (Japanese Patent Laying-Open No. 2009-81406) discloses a technique for forming a buffer layer on a substrate by the reactive sputtering method, and forming a group III nitride semiconductor layer thereon by the MOVPE method. Patent Literature 6 describes that, preferably, the buffer layer contains oxygen, and the buffer layer has an oxygen concentration of not more than 1 atomic % (paragraph [0028] of Patent Literature 6). Patent Literature 6 describes that, this seems to be because, if the oxygen concentration in the buffer layer is more than 1 atomic %, the buffer layer has too much oxygen, and thus consistency in lattice constant between the substrate and the buffer layer is reduced, and the function as a buffer layer is deteriorated (paragraph [0028] of Patent Literature 6).
Patent Literature 6 also describes that the reason why oxygen is contained in the buffer layer formed by the reactive sputtering method is that an oxygen-containing substance such as moisture sticking to an inner wall of a chamber of a sputtering apparatus is knocked out of the inner wall by sputtering, and oxygen is inevitably mixed into the buffer layer stacked on the substrate (paragraph [0028] of Patent Literature 6).
In addition, in Patent Literature 6, in order to set the oxygen concentration in the buffer layer to not more than 1 atomic %, dummy discharge is repeated 16 times within the chamber before the formation of the buffer layer to reduce a pressure within the chamber such that its internal pressure is reduced to 6×10−6 Pa, to remove impurities within the chamber.    PTL 1: Japanese Patent No. 3026087    PTL 2: Japanese Patent Publication No. 5-86646    PTL 3: Japanese Patent No. 3440873    PTL 4: Japanese Patent No. 3700492    PTL 5: Japanese Patent Laying-Open No. 2006-4970    PTL 6: Japanese Patent Laying-Open No. 2009-81406